LETTER
Palladium-Catalyzed Homocoupling of Arylboronic Acid Derivatives
1029
Table 3 Recycling Experiments Using Pd(II) EnCat 30TMa
Moreover, the efficiency of the homocoupling of phenyl-
boronic acids bearing an electron-withdrawing group was
improved by using PdEnCat 30.
Run
1
2
3
4
5
1a Conversion (%)
2a Yield (%)
95
64
100
65
97
64
91
64
85
60
Acknowledgment
This research was supported by Grant-in-Aid for Young Scientist
(A) 17685008 from Ministry of Education, Culture, Sports, Science
and Technology, Japan.
a All runs were carried out with 3 mol% Pd(II) EnCat 30, 0.5 mmol
p-tolueneboronic acid (1a), 10 mL MeOH at 26 1 °C for 24 h under
air.
References and Notes
boronic acid 1e similarly underwent homocoupling to
give 2e in 60% yield (run 4). In contrast, both electron-do-
nating and -accepting substituents diminished the homo-
coupling efficiency. The yields of the corresponding
biaryls 2b,c,f,g,j ranged from 44% to 52% (runs 2,3,5–7).
The yields of the biaryls bearing an electron-accepting
substituent were, however, improved compared to those
obtained from the Pd(OAc)2-catalyzed reactions. For
instance, the yields of chlorinated phenylboronic acids 1f
and 1j were increased from ca. 30% with Pd(OAc)2 up
to ca. 50% with PdEnCat 30. It is also noteworthy that
p-ethoxycarbonylphenylboronic acid 1o was converted
into biaryl 2o in 52% yield without ester exchange.
(1) (a) Bringmann, G.; Menche, D. Acc. Chem. Res. 2001, 34,
615. (b) Lloyd-Williams, P.; Giralt, E. Chem. Soc. Rev.
2001, 30, 145.
(2) (a) Meier, H. Angew. Chem. Int. Ed. 2005, 44, 2482.
(b) Tian, H.; Yang, S. Chem. Soc. Rev. 2004, 33, 85.
(c) Lemieux, R. P. Acc. Chem. Res. 2001, 34, 845.
(3) Shimizu, H.; Nagasaki, I.; Saito, T. Tetrahedron 2005, 61,
5405.
(4) (a) Hassan, J.; Sévignon, M.; Gozzi, C.; Schulz, E.; Lemaire,
M. Chem. Rev. 2002, 102, 1359. (b) Stanforth, S. P.
Tetrahedron 1998, 54, 263.
(5) (a) Suzuki, A. J. Organomet. Chem. 1999, 576, 147.
(b) Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457.
(6) For selected examples, see: (a) Cravotto, G.; Beggiato, M.;
Penoni, A.; Palmisano, G.; Tollari, S.; Lévêque, J.-M.;
Bonrath, W. Tetrahedron Lett. 2005, 46, 2267. (b) Punna,
S.; Díaz, D. D.; Finn, M. G. Synlett 2004, 2351.
(c) Yoshida, H.; Yamaryo, Y.; Ohshita, J.; Kunai, A.
Tetrahedron Lett. 2003, 44, 1541. (d) Parrish, J. P.; Jung, Y.
C.; Floyd, R. J.; Jung, K. W. Tetrahedron Lett. 2002, 43,
7899. (e) Kabalka, G. W.; Wang, L. Tetrahedron Lett. 2002,
43, 3067. (f) Lei, A.; Zhang, X. Tetrahedron Lett. 2002, 43,
2525. (g) Wong, M. S.; Zhang, X. L. Tetrahedron Lett. 2001,
42, 4087.
(7) Recently, Au-catalyzed homocouplings were reported, see:
(a) Carrettin, S.; Guzman, J.; Corma, A. Angew. Chem. Int.
Ed. 2005, 44, 2242. (b) González-Arellano, C.; Corma, A.;
Iglesias, M.; Sánchez, F. Chem. Commun. 2005, 1990.
(c) Tsunoyama, H.; Sakurai, H.; Ichikuni, N.; Negishi, Y.;
Tsukuda, T. Langmuir 2004, 20, 11293.
Table 4 Homocouplinga of Arylboronic Acids Using Pd(II) EnCat
30TM
Run
1
1
Pd (mol%) Time (h)
2, Yield (%)
2a, 64
2b, 50
2c, 51
1a
1b
1c
1e
1f
1g
1j
1o
3
3
3
3
3
5
3
5
24
48
2
3
48
4
48
2e, 60
5
72
2f, 52
6
120
72
2g, 44
2j, 50
(8) (a) Yamamoto, Y.; Ishii, J.; Nishiyama, H.; Itoh, K. J. Am.
Chem. Soc. 2005, 127, 9625. (b)Yamamoto, Y.;Hattori, K.;
Ishii, J.; Nishiyama, H.; Itoh, K. Chem. Commun. 2005,
4438.
(9) Goodson, F. E.; Wallow, T. I.; Novak, B. M. J. Am. Chem.
Soc. 1997, 119, 12441.
7
8
72
2o, 52
a All reactions were carried out with 3 mol% or 5 mol% Pd(II) EnCat
30, 0.5 mmol of an arylboronic acid, 10 mL MeOH at 26 °C under air.
(10) Nishimura, T.; Kakiuchi, N.; Onoue, T.; Ohe, K.; Uemura, S.
J. Chem. Soc., Perkin Trans. 1 2000, 1915.
In conclusion, we developed a base- and ligand-free pal-
ladium(II)-catalyzed homocoupling of arylboronic acids,
leading to symmetrical biaryls. Upon treatment with 3–5
mol% Pd(OAc)2 in MeOH at room temperature under air,
phenylboronic acids bearing an electron-donating substit-
uent were converted to the corresponding biaryls in mod-
erate to excellent yields. In contrast, phenylboronic acids
possessing an electron-withdrawing substituent resulted
in lower yields.
(11) Typical Procedure for the Synthesis of 2a.
Method i: To a solution of p-tolylboronic acid (1a, 115.7 mg,
0.85 mmol) in MeOH (17 mL) was added Pd(OAc)2 (5.7 mg,
0.026 mmol), and the reaction mixture was stirred for 5 h at
r.t. under air. The reaction mixture was concentrated in
vacuo and the residue was diluted with EtOAc (15 mL).
After filtration of insoluble materials, the filtrate was
concentrated in vacuo and the residue was purified by silica
gel column flush chromatography (eluent: hexane) to afford
2a (72.6 mg, 94%) as colorless solids.
Method ii: To a solution of p-tolylboronic acid (1a, 68.0 mg,
0.50 mmol) in MeOH (10 mL) was added PdEnCat 30 (ca.
0.4 mmol/g, 37.5 mg, 0.015 mmol), and the reaction mixture
was stirred for 24 h at r.t. under air. After filtration of the
catalyst, the filtrate was concentrated in vacuo and the
residue was purified by silica gel column flush chromato-
In place of Pd(OAc)2, commercially available polyurea-
encapsulated Pd(OAc)2 catalysts, PdIIEnCatTM, were able
to promote homocoupling of arylboronic acids albeit in
moderate yields, and were recycled by simple filtration.
Synlett 2006, No. 7, 1027–1030 © Thieme Stuttgart · New York